La Crosse (LAC) encephalitis virus has been on the move, expanding its range from the traditional upper Midwest distribution into the Southeast. Also, newly introduced potential vectors are becoming established in endemic areas, possibly affecting the virus cycle. This study proposes to investigate the apparent emergence of LAC virus into areas of southwestern Virginia. Comparisons will be made between an area with a history of reported human cases and isolation of LAC virus from field-collected mosquitoes, an area with no human cases but several mosquito isolations, and an area lacking either indication of virus activity. Field studies were designed to determine the rate of infection of mosquitoes in the different areas. Emphasis will be placed on collection and virus testing (by PCR) of Ochlerotatus triseriatus, the primary vector, and Aedes albopictus, a suspected accessory vector. Preliminary studies showed tha Me. albopictus preferred urban/residential areas as opposed to Oc. triseriatus which preferred forested areas. Also, the majority of LAC-virus positive samples collected in 2002 were from Ae. albopictus. Thus, this mosquito could be acting to increase the risk of human exposure to LAC virus by bringing it into suburban areas with greater population density. The roles of Oc. canadensis, a known accessory vector of LAC in some areas, and Oc. japonicus, a nonnative species recently found to have established in LAC endemic areas, will also be examined. In cooperation with local veterinarians and animal shelters, a canine serosurvey will be done to measure horizontal transmission in each of the study areas. By comparing canine seroprevalence with mosquito infection rates and distribution of human cases, we hope to validate this method for measuring the incidence of LAC virus transmission in an area. Specific aims are: 1. Measure the mosquito species composition and relative density at multiple field sites in each of the 3 regions. 2. Calculate the minimum infection rates of all potential vectors at each site. 3. Compare mosquito infection rates between species, sites, and among regions. 4. Determine if mosquito prevalence and minimum infection rates can be associated with consistent habitat features across the three regions. 5. Determine if canine seropositivity correlates with mosquito infection rates and human cases.